The synthesis of high molecular, thermoplastic polycarbonates by the two-phase boundary process is described, for example, in H. Schnell, "Chemistry and Physics of Polycarbonates", New York-London-Sidney, Interscience Publishers, 1964, "Polymer Reviews," Volume 9, in U.S. Pat. No. 3,275,601, in German Offenlegungsschriften (German Published Specifications) Nos. 2,063,050 and 2,211,957 and in German patent applications P 29 01 668.0 (LeA 19,146) and P 29 01 665.7 (LeA 19,040).
In that synthesis, when the polycondensation reaction has ended, the two-phase mixtures which consist of an organic phase and an aqueous alkaline phase are obtained. The organic phase in general consists of one or more organic solvents, which contain the dissolved polycarbonate. The aqueous alkaline phase in general consists of a solution of alkali metal hydroxide and salts which are formed during the condensation reaction. If NaOH is used as the alkali metal hydroxide, NaCl and Na.sub.2 CO.sub.3, for example, formed during the condensation reaction, are present in the aqueous phase. In addition to the substances mentioned, catalysts such as, for example, tertiary amines or quaternary ammonium, arsonium and phosphonium compounds are, in general, present in one or both phases.
As is known, separation and purification of the resulting organic solutions in a prerequisite for isolating the synthesized polycarbonate.
The alkali metal hydroxide, the catalysts and the salts can be separated off from the organic solution of polycarbonate as follows:
1. Separation of the alkaline two-phase mixture by settling or centrifugation (discontinuously or continuously). If necessary, the separated off organic phase is mixed with pure dilute alkali metal hydroxide solution and is again separated to remove unreacted bis-phenolate. If necessary, the process is repeated. PA1 2. Mixing of the organic phase with aqueous acid in order to neutralize residual traces of alkali metal hydroxide and, if necessary, to remove residues of catalysts (for example, amines as ammonium salts). Separation is carried out as described under 1. Repetition of the operation, if necessary. PA1 3. Mixing of the organic phase with water in order to remove traces of salt and acid from the organic solution. Separation is carried out as described under 1. Repetition of the operation, if necessary.
The amount of dilute alkali metal hydroxide solution to remove unreacted bis-phenolate from the organic phase, the amount of aqueous acid to neutralize residual traces of alkali metal hydroxide and catalyst in the organic phase and the amount of water to remove traces of salt and acid from the organic phase are selected such that the volume ratio of aqueous phase to organic phase is between 0.01:1 to 5:1, the ratios of between 0.1:1 and 2:1 are preferred.
In general, at least the washing operation described under 3 must be repeated several times in order to obtain sufficiently purified polycarbonate solutions. This is related to the fact that the separation of the alkaline or acid to neutral two-phase mixtures does not take place quantitatively in practice, since even in the case of an efficient separation, the organic phase in general still contains droplets of the aqueous phase. Phase separations in the alkaline acid and neutral ranges in general depend on the raw materials and reaction conditions employed for the polycarbonate synthesis. In many cases, the phase separation is so poor that sufficient purification of the polycarbonate solution is not possible even if great losses of material were acceptable and is, in fact, impossible.